Semiconductor device including insulated gate type transistor with pocket regions and insulated gate type capacitor with no region of reverse conductivity type

ABSTRACT

It is an object to obtain a semiconductor device having such a structure that respective electrical characteristics of an insulated gate type transistor and an insulated gate type capacitance are not deteriorated and a method of manufacturing the semiconductor device. An NMOS transistor Q 1  and a PMOS transistor Q 2  which are formed in an NMOS formation region A 1  and a PMOS formation region A 2  respectively have P 31   pocket regions  17  and N 31   pocket regions  27  in vicinal regions of extension portions  14   e  and  24   e  of N +  source-drain regions  14  and P +  source-drain regions  24 , respectively. On the other hand, an N-type variable capacitance C 1  and a P-type variable capacitance C 2  which are formed in an N-type variable capacitance formation region A 3  and a P-type variable capacitance formation region A 4  respectively do not have a region of a reverse conductivity type which is adjacent to extraction electrode regions corresponding to the P 31   pocket regions  17  and the N 31   pocket regions  27.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having an insulated gate type transistor and an insulated gate type capacitance and a method of manufacturing the semiconductor device.

2. Description of the Background Art

In a transistor having a gate length of a subquarter micron or less, a pocket injecting process for forming pocket regions is executed in order to suppress a short channel effect. The pocket injection is also referred to as NUDC (Non Uniformly Doped Channel).

FIG. 36 is a sectional view showing the pocket injecting process. As shown in FIG. 36, in the formation of a CMOS transistor, an NMOS formation region A11 and a PMOS formation region A12 are isolated from each other through an isolating film 102 provided in an upper layer portion of a semiconductor substrate 101.

In the NMOS formation region A11, a gate oxide film 112 and a gate electrode 113 are formed on a surface of a P well region 111 and a P-type impurity ion 103 is implanted and diffused by using the gate electrode 113 as a mask. Consequently, a P-type impurity implantation region 119 to be pocket regions of an NMOS transistor is formed.

In the PMOS formation region A12, similarly, a gate oxide film 122 and a gate electrode 123 are formed on a surface of an N well region 121 and an N-type impurity ion 104 is implanted and diffused by using the gate electrode 123 as a mask. Consequently, an N-type impurity implantation region 129 to be pocket regions of a PMOS transistor is formed.

More specifically, in the pocket injecting process, an impurity of the same conductivity type as that of a channel region of each MOS transistor is implanted into each of the NMOS formation region A11 and the PMOS formation region A12. In the pocket injecting process, the distribution of an impurity in a direction of a channel length becomes nonuniform and an effective channel impurity concentration is increased when a gate length becomes smaller. As a result, it is possible to suppress the short channel effect.

FIG. 37 is a sectional view showing a state in which a CMOS transistor is finished after the pocket injecting process.

As shown in FIG. 37, in the NMOS formation region A11, N⁺ source-drain regions 114 and 114 are formed to interpose a channel region provided under the gate electrode 113 and tip regions opposed to each other between the N⁺ source-drain regions 114 and 114 are extension portions 114 e, respectively.

In a vicinal region of the extension portion 114 e, the P-type impurity implantation region 119 remains as P³¹ pocket regions 117 from the extension portion 114 e to a part of the channel region. Moreover, side walls 116 and 116 are formed on both side surfaces of the gate electrode 113, respectively.

Thus, an NMOS transistor Q11 is formed by the gate oxide film 112, the gate electrode 113, the N⁺ source-drain regions 114, the side wall 116 and the P³¹ pocket regions 117.

In the PMOS formation region A12, P⁺ source-drain regions 124 and 124 are formed to interpose a channel region provided under the gate electrode 123 and tip regions opposed to each other between the P⁺ source-drain regions 124 and 124 are extension portions 124 e, respectively.

In a vicinal region of the extension portion 124 e, the N-type impurity implantation region 129 remains as N³¹ pocket regions 127 from the extension portion 124 e to a part of the channel region. Moreover, side walls 126 and 126 are formed on both side surfaces of the gate electrode 123, respectively.

Thus, a PMOS transistor Q12 is formed by the gate oxide film 122, the gate electrode 123, the P⁺ source-drain regions 124, the side wall 126 and the N³¹ pocket regions 127.

On the other hand, in a high-frequency analog circuit or a high-speed digital circuit, it is necessary to manufacture an LC type VCO (Voltage Controlled Oscillator) by using an inductor (L) and a variable capacitance (C).

In the case in which the variable capacitance to be an insulated gate type capacitance which has a small loss is to be obtained by utilizing the structure of the MOS transistor, it is necessary to generate an accumulation type variable capacitance in which impurities in a substrate (a body region) and extraction electrode portions have the same conductivity type.

FIG. 38 is a sectional view showing a structure of the accumulation type variable capacitance. As shown in FIG. 38, in the formation of the accumulation type variable capacitance, a P-type variable capacitance formation region A13 and an N-type variable capacitance formation region A14 are isolated from each other through an isolating film 102 provided in an upper layer portion of a semiconductor substrate 101.

In the P-type variable capacitance formation region A13, P⁺ extraction electrode regions 134 and 134 are formed to interpose a channel region provided under a gate electrode 133 and tip regions opposed to each other between the P⁺ extraction electrode regions 134 and 134 are extension portions 134 e, respectively.

In a vicinal region of the extension portion 134 e, N³¹ pocket regions 137 are formed from the extension portion 134 e to a part of the channel region. Moreover, side walls 136 and 136 are formed on both side surfaces of the gate electrode 133, respectively.

Thus, a P-type variable capacitance C11 is formed by a gate oxide film 132, the gate electrode 133, the P⁺ extraction electrode regions 134, the side wall 136 and the N³¹ pocket regions 137. In other words, the P-type variable capacitance C11 acts as an insulated gate type capacitance in which the P⁺ extraction electrode regions 134 are set to one of electrodes, the gate electrode 133 is set to the other electrode and the gate oxide film 132 is set to an interelectrode insulating film.

In the N-type variable capacitance formation region A14, N⁺ extraction electrode regions 144 and 144 are formed to interpose a channel region provided under a gate electrode 143 and tip regions opposed to each other between the N⁺ extraction electrode regions 144 and 144 are extension portions 144 e, respectively.

In a vicinal region of the extension portion 144 e, P³¹ pocket regions 147 are formed from the extension portion 144 e to a part of the channel region. Moreover, side walls 146 and 146 are formed on both side surfaces of the gate electrode 143, respectively.

Thus, an N-type variable capacitance C12 is formed by a gate oxide film 142, the gate electrode 143, the N⁺ extraction electrode regions 144, the side wall 146 and the P³¹ pocket regions 147. In other words, the N-type variable capacitance C12 acts as an insulated gate type capacitance in which the N⁺ extraction electrode regions 144 are set to one of electrodes, the gate electrode 143 is set to the other electrode and the gate oxide film 142 is set to an interelectrode insulating film.

FIGS. 39 and 40 are views illustrating a degree of a change in a capacitance value of the N-type variable capacitance C12. In the case in which a gate voltage VG to be applied to the gate electrode 143 is lower than 0 V, a depletion layer 148 is extended downward in an N well region 121 provided under the gate electrode 143 as shown in FIG. 39 so that a capacitance value of the N-type variable capacitance C12 is decreased. On the other hand, in the case in which the gate electrode VG is higher than 0 V, the depletion layer 148 is reduced in the N well region 121 provided under the gate electrode 143 as shown in FIG. 40 so that the capacitance value of the N-type variable capacitance C12 is increased. Thus, it is possible to variably set the capacitance value of the N-type variable capacitance C12 based on the gate voltage VG to be applied to the gate electrode 143. Also in the P-type variable capacitance C11, similarly, it is possible to change the capacitance value based on the gate voltage to be applied to the gate electrode 133.

However, when the pocket injecting process shown in FIG. 36 is executed in order to enhance a short channel characteristic (to suppress the short channel effect), pocket regions of a reverse conductivity type to that of the body region are formed with the accumulation type variable capacitance in extraction electrode regions and the body region to be a region of the semiconductor substrate 101 which is provided just below the gate electrode. Therefore, there has been a problem in that a series resistance is increased.

FIG. 41 is a circuit diagram showing an equivalent circuit of the variable capacitance in FIG. 38. As shown in FIG. 41, the variable capacitance is equivalently represented by a series connection of a capacitance component C10 and a resistance component R10.

On the other hand, an index representing an electrical characteristic of the variable capacitance includes a Q—factor (Q-value). The Q-value is expressed in the following equation (1), wherein Q represents a Q-value, ω represents an angular frequency, C represents a capacitance value of the capacitance component C10 and R represents a resistance value of the resistance component R10.

[Equation  1] $\begin{matrix} {Q = \frac{1}{\omega\;{CR}}} & (1) \end{matrix}$

When the Q-value is increased, an energy efficiency of the capacitance is enhanced. There has been a problem in that the resistance value R of the resistance component R10 is increased by the presence of the pocket regions so that the Q-value is decreased in accordance with the equation (1).

SUMMARY OF THE INVENTION

It is an object of the present invention to obtain a semiconductor device having such a structure that respective electrical characteristics of an insulated gate type transistor and an insulated gate type capacitance are not deteriorated and a method of manufacturing the semiconductor device.

A first aspect of the present invention, a semiconductor device includes an insulated gate type transistor and an insulated gate type capacitance which are formed in a semiconductor substrate. The insulated gate type transistor includes a gate insulating film, a gate electrode and source-drain regions. The gate insulating film for a transistor is selectively formed on the semiconductor substrate. The gate electrode for a transistor is formed on the gate insulating film for a transistor. The source-drain regions are formed to interpose a body region for a transistor provided under the gate electrode for a transistor in a surface of the semiconductor substrate. The insulated gate type capacitance includes a gate insulating film, a gate electrode and extraction electrode regions. The gate insulating film for a capacitance is selectively formed on the semiconductor substrate. The gate electrode for a capacitance is formed on the gate insulating film for a capacitance. The extraction electrode regions are formed to interpose a body region for a capacitance provided under the gate electrode for a capacitance in the surface of the semiconductor substrate. The insulated gate type transistor has pocket regions for a transistor of a reverse conductivity type to that of the source-drain regions formed from the source-drain regions to a part of the body region for a transistor. The insulated gate type capacitance has no region of a reverse conductivity type to that of the extraction electrode regions in a vicinal region of the extraction electrode regions in the body region side for a capacitance.

The insulated gate type transistor of the semiconductor device has the pocket regions for a transistor. Consequently, a short channel effect can be suppressed.

On the other hand, the insulated gate type capacitance does not have the region of the reverse conductivity type to that of the extraction electrode regions (a region of a reverse conductivity type which is adjacent to the extraction electrode regions) in the vicinal region of the extraction electrode regions on the body region side for a capacitance. Therefore, an electrical characteristic can be prevented from being deteriorated by the presence of the region of a reverse conductivity type which is adjacent to the extraction electrode regions.

As a result, it is possible to obtain a semiconductor device having such a structure that the respective electrical characteristics of the insulated gate type transistor and the insulated gate type capacitance are not deteriorated.

A second aspect of the present invention, a semiconductor device includes an insulated gate type transistor and an insulated gate type capacitance which are formed in a semiconductor substrate. The insulated gate type transistor includes a gate insulating film, a gate electrode, source-drain regions and pocket regions. The gate insulating film for a transistor is selectively formed on the semiconductor substrate. The gate electrode for a transistor is formed on the gate insulating film for a transistor. The source-drain regions are formed to interpose a body region for a transistor provided under the gate electrode for a transistor in a surface of the semiconductor substrate. The pocket regions for a transistor of a reverse conductivity type to that of the source-drain regions are formed from the source-drain regions to a part of the body region for a transistor. The insulated gate type capacitance includes a gate insulating film, a gate electrode, extraction electrode regions and pocket regions. The gate insulating film for a capacitance is selectively formed on the semiconductor substrate. The gate electrode for a capacitance is formed on the gate insulating film for a capacitance. The extraction electrode regions are formed to interpose a body region for a capacitance provided under the gate electrode for a capacitance in the surface of the semiconductor substrate. The pocket regions for a capacitance of a reverse conductivity type to that of the extraction electrode regions are formed from the extraction electrode regions to a part of the body region for a capacitance. An impurity concentration in a surface of the body region for a capacitance is different from that in a surface of the body region for a transistor.

The impurity concentration in the surface of the body region for a capacitance and the impurity concentration in the surface of the body region for a transistor are set to be different from each other. Therefore, it is possible to enhance a degree of freedom of a design in the device.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a first embodiment of the present invention,

FIGS. 2 to 7 are sectional views showing a method of manufacturing the semiconductor device according to the first embodiment,

FIG. 8 is a sectional view showing a structure of a semiconductor device according to a second embodiment of the present invention,

FIG. 9 is a view illustrating the influence of a high frequency current of an N-type variable capacitance according to the second embodiment,

FIG. 10 is a view illustrating the influence of a high frequency current of an N-type variable capacitance having pocket regions formed in an SOI substrate,

FIG. 11 is a sectional view showing a structure of a semiconductor device according to a third embodiment of the present invention,

FIGS. 12 to 15 are sectional views showing a method of manufacturing a semiconductor device according to a fourth embodiment,

FIG. 16 is a sectional view showing a structure of a semiconductor device according to a fifth embodiment of the present invention,

FIG. 17 is a sectional view showing a structure of a semiconductor device according to a sixth embodiment of the present invention,

FIG. 18 is a diagram illustrating a layout structure of a semiconductor device according to a seventh embodiment of the present invention,

FIG. 19 is a sectional view showing a structure of a high voltage transistor formation region and a variable capacitance formation region in the semiconductor device according to the seventh embodiment,

FIG. 20 is a sectional view showing a structure of a semiconductor device according to an eighth embodiment of the present invention,

FIG. 21 is a sectional view showing a part of a method of manufacturing a semiconductor device having a MOS transistor and a variable capacitance according to a ninth embodiment of the present invention,

FIGS. 22 to 26 are sectional views showing a method of manufacturing an N-type variable capacitance in a semiconductor device according to a tenth embodiment,

FIG. 27 is a sectional view showing a structure of a semiconductor device according to an eleventh embodiment of the present invention,

FIG. 28 is a sectional view showing a structure of a first mode of a semiconductor device according to a twelfth embodiment of the present invention,

FIG. 29 is a sectional view showing a structure of a second mode of the semiconductor device according to the twelfth embodiment of the present invention,

FIG. 30 is a sectional view showing a structure of a first mode of a semiconductor device according to a thirteenth embodiment of the present invention,

FIGS. 31 to 35 are sectional views showing a method of forming a porous silicon layer,

FIGS. 36 and 37 are sectional views showing a method of manufacturing a MOS transistor having pocket regions according to a conventional example,

FIG. 38 is a sectional view showing a structure of a variable capacitance,

FIGS. 39 and 40 are views illustrating a capacitance value setting operation for a variable capacitance, and

FIG. 41 is a circuit diagram showing an equivalent circuit of the variable capacitance illustrated in FIG. 38.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<First Embodiment>

FIG. 1 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a first embodiment of the present invention.

As shown in FIG. 1, an NMOS transistor Q1, a PMOS transistor Q2, an N-type variable capacitance C1 and a P-type variable capacitance C2 are formed in an NMOS formation region A1, a PMOS formation region A2, an N-type variable capacitance formation region A3 and a P-type variable capacitance formation region A4, respectively. Each of the formation regions A1 to A4 is isolated by an isolating film (not shown) or the like. Moreover, well regions 11, 21, 31 and 41 to be body regions are formed in the formation regions A1, A2, A3 and A4, respectively.

In the NMOS formation region A1, a gate oxide film 12 is selectively formed on a surface of the P well region 11 and an N⁺-type gate electrode 13 is formed on the gate oxide film 12. N⁺ source-drain regions 14 and 14 are formed to interpose a channel region to be a surface region of the P well region 11 provided under the gate electrode 13, and tip regions protruded and opposed to each other between the N⁺ source-drain regions 14 and 14 are extension portions 14 e, respectively.

In a vicinal region of the extension portion 14 e, P³¹ pocket regions 17 are formed from the extension portion 14 e to a part of the channel region. Moreover, side walls 16 and 16 are formed on both side surfaces of the gate electrode 13, respectively. Furthermore, a silicide region 14 s and a silicide region 13 s are formed in a surface of the N⁺ source-drain regions 14 and an upper layer portion of the gate electrode 13, respectively.

Thus, the NMOS transistor Q1 is formed by the gate oxide film 12, the gate electrode 13, the N⁺ source-drain regions 14, the side wall 16 and the P³¹ pocket regions 17.

In the PMOS formation region A2, a gate oxide film 22 is selectively formed on a surface of the N well region 21 and a P⁺-type gate electrode 23 is formed on the gate oxide film 22. P⁺ source-drain regions 24 and 24 are formed to interpose a channel region provided under the gate electrode 23, and tip regions protruded and opposed to each other between the P⁺ source-drain regions 24 and 24 are extension portions 24 e, respectively.

In a vicinal region of the extension portion 24 e, N³¹ pocket regions 27 are formed from the extension portion 24 e to a part of the channel region. Moreover, side walls 26 and 26 are formed on both side surfaces of the gate electrode 23, respectively. Furthermore, a silicide region 24 s and a silicide region 23 s are formed in a surface of the P⁺ source-drain regions 24 and an upper layer portion of the gate electrode 23, respectively.

Thus, a PMOS transistor Q2 is formed by the gate oxide film 22, the gate electrode 23, the P⁺ source-drain regions 24, the side wall 26 and the N³¹ pocket regions 27.

In the N-type variable capacitance formation region A3, a gate oxide film 32 is selectively formed on a surface of the N well region 31 and an N⁺-type gate electrode 33 is formed on the gate oxide film 32. N⁺ extraction electrode regions 34 and 34 are formed to interpose a body surface region to be a surface of the N well region 31 provided under the gate electrode 33. Tip regions protruded and opposed to each other between the N⁺ extraction electrode regions 34 and 34 are extension portions 34 e, respectively.

Moreover, side walls 36 and 36 are formed on both side surfaces of the gate electrode 33, respectively. Furthermore, a silicide region 34 s and a silicide region 33 s are formed in a surface of the N⁺ extraction electrode regions 34 and an upper layer portion of the gate electrode 33, respectively.

Thus, an N-type (N⁺ gate/N³¹ body type) variable capacitance C1 is formed by the gate oxide film 32, the gate electrode 33, the N⁺ extraction electrode regions 34 and the side wall 36. More specifically, the N-type variable capacitance C1 becomes an insulated gate type capacitance in which the N⁺ extraction electrode regions 34 are set to one of electrodes, the gate electrode 33 is set to the other electrode and the gate oxide film 32 is set to an interelectrode insulating film. By a gate voltage to be applied to the gate electrode 33, it is possible to variably set a capacitance value by changing the extension of a depletion layer in the N well region 31 provided under the gate electrode 33.

In the P-type variable capacitance formation region A4, a gate oxide film 42 is selectively formed on a surface of the P well region 41 and a P⁺-type gate electrode 43 is formed on the gate oxide film 42. P⁺ extraction electrode regions 44 and 44 are formed to interpose a body surface region provided under the gate electrode 43. Tip regions protruded and opposed to each other between the P⁺ extraction electrode regions 44 and 44 are extension portions 44 e, respectively.

Moreover, side walls 46 and 46 are formed on both side surfaces of the gate electrode 43, respectively. Furthermore, a silicide region 44 s and a silicide region 43 s are formed in a surface of the P⁺ extraction electrode region 44 and an upper layer portion of the gate electrode 43, respectively.

Thus, a P-type (P⁺ gate/P³¹ body type) variable capacitance C2 is formed by the gate oxide film 42, the gate electrode 43, the P⁺ extraction electrode regions 44 and the side wall 46. More specifically, the P-type variable capacitance C2 becomes an insulated gate type capacitance in which the P⁺ extraction electrode regions 44 are set to one of electrodes, the gate electrode 43 is set to the other electrode and the gate oxide film 42 is set to an interelectrode insulating film. By a gate voltage to be applied to the gate electrode 43, it is possible to variably set a capacitance value by changing the extension of a depletion layer in the P well region 41 provided under the gate electrode 43.

As described above, in the semiconductor device according to the first embodiment, the pocket regions are present in the MOS transistor. Therefore, it is possible to obtain a MOS transistor in which a short channel effect is suppressed.

On the other hand, pocket regions (regions of a reverse conductivity type which are adjacent to extraction electrode regions) are not present in the variable capacitance. More specifically, the region of a reverse conductivity type to that of the extraction electrode regions is not present at all in a region provided in the vicinity of the extraction electrode regions of the variable capacitance. Therefore, a series resistance is low and a Q-value is not deteriorated.

As the semiconductor device according to the first embodiment, thus, it is possible to obtain a semiconductor device comprising a MOS transistor suppressing a short channel effect and a variable capacitance in which a series resistance is low and a Q-value is not deteriorated.

FIGS. 2 to 7 are sectional views showing a method of manufacturing the semiconductor device according to the first embodiment. A procedure for manufacturing the semiconductor device according to the first embodiment will be described below with reference to these drawings.

As shown in FIG. 2, first of all, a P well region 11, an N well region 21, an N well region 31 and a P well region 41 which are to be body regions are formed, by an existing method, in an NMOS formation region A1, a PMOS formation region A2, an N-type variable capacitance formation region A3 and a P-type variable capacitance formation region A4 which are isolated from each other. Then, a gate oxide film 12 and an N⁺-type gate electrode 13 are selectively formed on a surface of the P well region 11, a gate oxide film 22 and a P⁺-type gate electrode 23 are selectively formed on a surface of the N well region 21, a gate oxide film 32 and an N⁺-type gate electrode 33 are selectively formed on a surface of the N well region 31, and a gate oxide film 42 and a P⁺-type gate electrode 43 are selectively formed on a surface of the P well region 41.

As shown in FIG. 3, a resist 51 is formed on a region other than the NMOS formation region A1, and a P-type impurity ion 61 and an N-type impurity ion 62 are then implanted and diffused sequentially into only the NMOS formation region A1 by varying an implantation energy with the gate electrode 13 to be a mask. Thus, a P³¹ diffusion region 19 and an N³¹ extension region 18 are formed, respectively.

As a specific example of the implantation of the N-type impurity ion 62, it can be proposed that an arsenic ion is implanted at an implantation energy of 3 to 20 keV, a dose of 1×10¹⁴ to 1×10¹⁵/cm², and an implantation angle of 0 degree.

As a specific example of the implantation of the P-type impurity ion 61, moreover, it can be proposed that a boron ion is implanted at an implantation energy of 10 to 20 keV, a dose of 1×10¹³ to 3×10¹³/cm², and an implantation angle of 0 to 45 degrees.

As shown in FIG. 4, subsequently, the resist 51 is removed and a resist 52 is then formed on a region other than the N-type variable capacitance formation region A3 and an N-type impurity ion 63 is thereafter implanted into only the N-type variable capacitance formation region A3 with the same contents as those of the implantation of the N-type impurity ion 62, for example. Consequently, an N³¹ extension region 38 is formed. In this case, it can also be proposed that an N-type impurity ion is implanted on the same conditions as those of an N-type impurity ion 64 which will be described below, thereby further forming N³¹ pocket regions.

As shown in FIG. 5, next, the resist 52 is removed and a resist 53 is then formed on a region other than the PMOS formation region A2, and the N-type impurity ion 64 and a P-type impurity ion 65 are then implanted and diffused sequentially into only the PMOS formation region A2 by varying an implantation energy with the gate electrode 23 to be a mask. Thus, an N³¹ diffusion region 29 and a P³¹ extension region 28 are formed.

As a specific example of the implantation of the P-type impurity ion 65, moreover, it can be proposed that a BF₂ ion is implanted at an implantation energy of 3 to 10 keV, a dose of 1×10¹⁴ to 1×10¹⁵/cm², and an implantation angle of 0 degree.

As a specific example of the implantation of the N-type impurity ion 64, furthermore, it can be proposed that an arsenic ion is implanted at an implantation energy of 50 to 150 keV, a dose of 1×10¹³ to 3×10¹³/cm², and an implantation angle of 0 to 45 degrees.

As shown in FIG. 6, subsequently, the resist 53 is removed and a resist 54 is then formed on a region other than the P-type variable capacitance formation region A4 and a P-type impurity ion 66 is thereafter implanted into only the P-type variable capacitance formation region A4 with the same contents as those of the implantation of the P-type impurity ion 65, for example. Consequently, a P³¹ extension region 48 is formed. In this case, it can also be proposed that a P-type impurity ion is implanted on the same conditions as those of the P-type impurity ion 61 to further form P³¹ pocket regions.

When the resist 54 is removed as shown in FIG. 7, it is possible to obtain such a structure that a P³¹ diffusion region 19 and an N³¹ diffusion region 29 which are to be pocket regions are present in only the MOS transistor formation regions A1 and A2 and a diffusion region to be the pocket regions is not present in the variable capacitance formation regions A3 and A4.

Subsequently, the structure shown in FIG. 1 can be obtained by using an existing method of forming a MOS transistor and a variable capacitance. In the structure shown in FIG. 1, a side wall is formed and source-drain regions (extraction electrode regions) are then formed, and furthermore, the inside of a surface of the source-drain regions (extraction electrode regions) and an upper layer portion of the gate electrode are silicided by a self-align silicide (salicide) process, thereby forming a silicide region and reducing a resistance.

As a specific example of the formation of the N⁺ source-drain regions 14 of the NMOS transistor Q1, it can be proposed that an arsenic ion is implanted at an implantation energy of 20 to 70 keV, a dose of 1×10¹⁵ to 1×10¹⁶/cm², and an implantation angle of 0 to 30 degrees.

As a specific example of the formation of the P⁺ source-drain regions 24 of the PMOS transistor Q2, moreover, it can be proposed that a BF₂ ion is implanted at an implantation energy of 10 to 30 keV, a dose of 1×10¹⁵ to 1×10¹⁶/cm², and an implantation angle of 0 to 30 degrees.

For example, CoSi₂, TiSi₂, NiSi₂ or the like is used for the silicide.

While both of the N- and P-type variable capacitances are formed in the present embodiment, only one of the variable capacitances may be formed. It is preferable that a variable capacitance of such a type as to be convenient for a circuit should be formed, and the N-type variable capacitance has a low resistance value of a series resistance component of a body portion and a great Q-value. In this respect, the N-type variable capacitance is more excellent.

<Second Embodiment>

FIG. 8 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a second embodiment of the present invention.

As shown in FIG. 8, a buried oxide film 4 is formed on a support substrate 3, and an SOI layer 5 formed on the buried oxide film 4 is isolated into an NMOS formation region A1, a PMOS formation region A2, an N-type variable capacitance formation region A3 and a P-type variable capacitance formation region A4 through an isolating film (not shown) or the like.

An NMOS transistor Q1, a PMOS transistor Q2, an N-type variable capacitance C1 and a P-type variable capacitance C2 which have the same structures as those of the first embodiment are formed in the NMOS formation region A1, the PMOS formation region A2, the N-type variable capacitance formation region A3 and the P-type variable capacitance formation region A4, respectively.

In the semiconductor device according to the second embodiment, thus, the MOS transistors Q1 and Q2 and the variable capacitances C1 and C2 which are similar to those of the first embodiment are formed on the SOI substrate (the support substrate 3, the buried oxide film 4 and the SOI layer 5). Accordingly, the structure and manufacturing method are the same as those in the first embodiment except that a bulk substrate is replaced with the SOI substrate.

FIG. 9 is a view illustrating the influence of a high frequency current of the N-type variable capacitance according to the second embodiment. As shown in FIG. 9, a high frequency current path CP1 mainly flows in the N-type variable capacitance C1 between N⁺ extraction electrode regions 34 in the vicinity of a gate oxide film 32 and a gate electrode 33. Therefore, a variable capacitance characteristic is not greatly deteriorated.

FIG. 10 is a view illustrating the influence of a high frequency current of an N-type variable capacitance having pocket regions formed in the SOI substrate. As shown in FIG. 10, a thickness of the SOI layer 5 is smaller than that of the bulk substrate so that a part (shown in a dotted line) of a high frequency current path CP2 flowing in an N-type variable capacitance C1P is made invalid and a series resistance is increased. Consequently, a degree of deterioration is great.

Thus, when P³¹ pocket regions 37 are present, the bad influence of the high frequency current path CP2 is increased. For the SOI structure, therefore, the structure according to the second embodiment in which the pocket regions are not provided in the variable capacitance is very effective.

<Third Embodiment>

FIG. 11 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a third embodiment of the present invention.

As shown in FIG. 11, variable capacitances C3 and C4 of an inversion type are provided in place of the variable capacitances C1 and C2 of an accumulation type. More specifically, the structure according to the third embodiment is different from the structure according to the first embodiment shown in FIG. 1 in that a P well region 30 is provided in place of the N well region 31 and an N well region 40 is provided in place of the P well region 41.

More specifically, the N-type (N⁺ gate/P³¹ body/N⁺S/D type) variable capacitance C3 and the P-type (P⁺ gate/N³¹ body/P⁺S/D type) variable capacitance C4 have structures equivalent to the structures of the NMOS transistor and the PMOS transistor, and are different from the NMOS transistor Q1 and the PMOS transistor Q2 in that pocket regions corresponding to the P³¹ pocket regions 17 and 27 are not provided.

In the variable capacitance having the inversion type structure, the pocket regions and the body region (the P well region 30, the N well region 40) have the same conductivity types even if the pocket regions are provided. Differently from the variable capacitance of the accumulation type, therefore, a resistance value of a series resistance component of the body region is not increased.

However, when the pocket regions are formed, an impurity concentration distribution in a direction of a channel length becomes nonuniform. Therefore, there is a problem in that a distribution in a direction of a channel is generated on a threshold voltage for a MOS transistor and it is hard to estimate a capacitance value of the variable capacitance based on a gate voltage.

By the structure according to the third embodiment in which the pocket regions are not provided in the variable capacitance of the inversion type, accordingly, it is possible to obtain an effect that precision in estimation of the capacitance value of the variable capacitance can be enhanced.

<Fourth Embodiment>

A fourth embodiment provides a method of manufacturing a semiconductor device which is different from the method of obtaining the structure according to the first embodiment. While the steps of forming the extension regions of the MOS transistor and the variable capacitance are carried out independently in the manufacturing method according to the first embodiment, a plurality of extension regions can be formed at the same time on the same conditions in the fourth embodiment.

FIGS. 12 to 15 are sectional views showing the method of manufacturing a semiconductor device according to the fourth embodiment. With reference to these drawings, description will be given to a procedure for manufacturing a semiconductor device according to the fourth embodiment.

First of all, the structure shown in FIG. 2 is obtained in the same manner as in the first embodiment. As shown in FIG. 12, then, a resist 55 is formed on a region other than an NMOS formation region A1 and an N-type variable capacitance formation region A3, and an N-type impurity ion 67 is thereafter implanted into only the NMOS formation region A1 and the N-type variable capacitance formation region A3 by using a gate electrode 13 and a gate electrode 33 as masks. Consequently, an N³¹ extension region 18 and an N³¹ extension region 38 are obtained at the same time. The N-type impurity ion 67 is implanted at an implantation angle of “0”.

As shown in FIG. 13, subsequently, a resist 56 is formed on a region other than the NMOS formation region A1 and a P-type impurity ion 68 is then implanted and diffused into only the NMOS formation region A1 by using the gate electrode 13 as a mask. Consequently, a P³¹ diffusion region 19 is formed. The P-type impurity ion 68 is obliquely implanted at a higher implantation energy than that of the N-type impurity ion 67.

As shown in FIG. 14, then, a resist 57 is formed on a region other than a PMOS formation region A2 and a P-type variable capacitance formation region A4, and a P-type impurity ion 69 is thereafter implanted into only the PMOS formation region A2 and the P-type variable capacitance formation region A4 by using a gate electrode 23 and a gate electrode 43 as masks. Consequently, a P³¹ extension region 28 and a P³¹ extension region 48 are obtained at the same time. The P-type impurity ion 69 is implanted at an implantation angle of “0”.

As shown in FIG. 15, subsequently, a resist 58 is formed on a region other than the PMOS formation region A2 and an N-type impurity ion 70 is then implanted and diffused into only the PMOS formation region A2 by using the gate electrode 23 as a mask. Consequently, an N³¹ diffusion region 29 is formed. The N-type impurity ion 70 is obliquely implanted at a higher implantation energy than that of the P-type impurity ion 69.

Subsequently, it is possible to obtain the structure shown in FIG. 1 by using the existing method of forming a MOS transistor and a variable capacitance.

In the method of manufacturing a semiconductor device according to the fourth embodiment, thus, the extension region is simultaneously formed in the MOS transistor and the variable capacitance which have the same conductivity type. As compared with the method of manufacturing a semiconductor device according to the first embodiment, therefore, two ion implanting steps can be eliminated.

<Fifth Embodiment>

FIG. 16 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a fifth embodiment of the present invention.

As shown in FIG. 16, variable capacitances C5 and C6 having no extension region are provided in place of the variable capacitances C1 and C2 having the extension regions. More specifically, as compared with the structure according to the first embodiment shown in FIG. 1, the N⁺ extraction electrode regions 34 having the extension portion 34 e is replaced with an N⁺ extraction electrode regions 35 having no extension portion and the P⁺ extraction electrode regions 44 having the extension portion 44 e is replaced with a P⁺ extraction electrode regions 45 having no extension portion. Other structures are the same as those of the first embodiment shown in FIG. 1.

A method of manufacturing a semiconductor device according to the fifth embodiment is different from the method of manufacturing a semiconductor device according to the first embodiment in that the steps of manufacturing the N³¹ extension region 38 and the P³¹ extension region 48 shown in FIGS. 4 and 6 are omitted.

In the method of manufacturing a semiconductor device according to the fifth embodiment, thus, the step of forming the extension region of a variable capacitance is omitted. Consequently, two steps, that is, the step of forming a resist and the step of implanting an ion can be eliminated as compared with the method of manufacturing a semiconductor device according to the first embodiment.

Although the variable capacitance cannot produce the effect by the provision of the extension portion in the semiconductor device according to the fifth embodiment, the effect of providing no pocket region can be obtained in the same manner as that in each of the first to fourth embodiments.

<Sixth Embodiment>

FIG. 17 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a sixth embodiment of the present invention.

As shown in FIG. 17, variable capacitances C7 and C8 having extension regions in higher concentrations than those of the variable capacitances C1 and C2 are provided in place of the variable capacitances C1 and C2.

More specifically, as compared with the structure according to the first embodiment, the N³¹ extension region 38 and the P³¹ extension region 48 shown in FIGS. 4 and 6 are formed to have impurity concentrations which are approximately twice to 100 times as high as those of the N³¹ extension region 18 and the P³¹ extension region 28 in the MOS transistor.

Although gate lengths of MOS transistors Q1 and Q2 are almost equal to those of the variable capacitances C7 and C8 in FIG. 17, the gate length of the variable capacitance is actually set to be greater than that of the MOS transistor in many cases.

Accordingly, the influence of a short channel effect in the variable capacitance is smaller than that in the MOS transistor. Consequently, a bad effect is lessened with an increase in the impurity concentration of the extension region. To the contrary, it is possible to obtain a greater advantage that a series resistance component of the variable capacitance can be reduced with the increase in the impurity concentration of the extension region.

While the example in which the extension region is formed in a high concentration has been described in the present embodiment, the same effects can be obtained even if an implantation energy of an impurity ion is increased to be approximately 1.2 to 30 times as high as that of the MOS transistor and a depth of the extension region is increased to be approximately 1.2 to 30 times as great as that of the MOS transistor.

<Seventh Embodiment>

In general, if a device is subjected to scaling, a power supply voltage is also subjected to the scaling and is thereby reduced. Consequently, it is necessary to provide an interface with another chip (device) to be operated at a high voltage.

At this time, it is necessary to fabricate a 3.3 V or 5.0 V compatible MOS transistor for a high voltage (hereinafter referred to as a “transistor for a high voltage”), for example, in addition to a high performance MOS transistor (hereinafter referred to as a “high performance transistor”) which is subjected to the scaling in the device.

By a comparison of the transistor for a high voltage with the high performance transistor, a gate length is greater and a gate oxide film has a greater thickness, and furthermore, an extension region is formed on different conditions and pocket regions are not formed in many cases. The extension region is formed on the different conditions in order to increase a hot carrier tolerance such that an S/D breakdown phenomenon such as punch-through is not presented at a high voltage. The pocket regions do not need to be formed because the gate length is great.

FIG. 18 is a diagram illustrating a layout structure of a semiconductor device having a MOS transistor and a variable capacitance according to a seventh embodiment of the present invention. As shown in FIG. 18, the semiconductor device is constituted by a high performance transistor formation region E1, a high voltage transistor formation region E2 and a variable capacitance formation region E3 according to the seventh embodiment, and a high performance transistor, a transistor for a high voltage and a variable capacitance are provided in the formation regions E1 to E3, respectively.

FIG. 19 is a sectional view showing structures of the high voltage transistor formation region E2 and the variable capacitance formation region E3 in the semiconductor device according to the seventh embodiment.

In an NMOS formation region A5 of the high voltage transistor formation region E2, a gate oxide film 72 is selectively formed on a surface of a P well region 71 to be a body region and an N⁺-type gate electrode 73 is formed on the gate oxide film 72. N⁺ source-drain regions 74 and 74 are formed to interpose a channel region provided under the gate electrode 73 and tip regions opposed to each other between the N⁺ source-drain regions 74 and 74 are extension portions 74 e, respectively.

Moreover, side walls 76 and 76 are formed on both side surfaces of the gate electrode 73, respectively. Furthermore, a silicide region 74 s and a silicide region 73 s are formed in a surface of the N⁺ source-drain regions 74 and an upper layer portion of the gate electrode 73, respectively.

Thus, an NMOS transistor Q3 for a high voltage is formed by the gate oxide film 72, the gate electrode 73, the N⁺ source-drain regions 74 and the side wall 76.

In a PMOS formation region A6 of the high voltage transistor formation region E2, a gate oxide film 82 is selectively formed on a surface of an N well region 81 and a P⁺-type gate electrode 83 is formed on the gate oxide film 82. P⁺ source-drain regions 84 and 84 are formed to interpose a channel region provided under the gate electrode 83 and tip regions opposed to each other between the P⁺ source-drain regions 84 and 84 are extension portions 84 e, respectively.

Moreover, side walls 86 and 86 are formed on both side surfaces of the gate electrode 83, respectively. Furthermore, a silicide region 84 s and a silicide region 83 s are formed in a surface of the P⁺ source-drain regions 84 and an upper layer portion of the gate electrode 83, respectively.

Thus, a PMOS transistor Q4 for a high voltage is formed by the gate oxide film 82, the gate electrode 83, the P⁺ source-drain regions 84 and the side wall 86.

The high performance transistor formed in the high performance transistor formation region E1 is provided to have the same structure as that of each of the NMOS transistor Q1 and the PMOS transistor Q2 according to the first embodiment shown in FIG. 1, for example, which is not shown in FIG. 19.

The NMOS transistor Q3 for a high voltage and the PMOS transistor Q4 for a high voltage are different from the NMOS transistor Q1 and the PMOS transistor Q2 for a high performance in that gate lengths are greater, the gate oxide films have greater thicknesses, the extension regions are set on different conditions and the pocket regions are not formed.

On the other hand, an N-type variable capacitance C1 and a P-type variable capacitance C2 are formed in an N-type variable capacitance formation region A3 and a P-type variable capacitance formation region A4 in the variable capacitance formation region E3, respectively.

The N-type variable capacitance C1 and the P-type variable capacitance C2 have the same basic structures as those of the N-type variable capacitance C1 and the P-type variable capacitance C2 according to the first embodiment shown in FIG. 1.

In each of the N-type variable capacitance C1 and the P-type variable capacitance C2, an extension region is formed to have a higher impurity concentration than that of the extension region of each of the NMOS transistor Q1 and the PMOS transistor Q2 and an equal gate length on the same conditions (at least the impurity concentration is almost equal) as compared with the NMOS transistor Q3 for a high voltage and the PMOS transistor Q4 for a high voltage. In the N-type variable capacitance C1 and the P-type variable capacitance C2, moreover, the gate oxide films have equal thicknesses as compared with the NMOS transistor Q1 and the PMOS transistor Q2.

In the semiconductor device having such a structure according to the seventh embodiment, the extension regions in the NMOS transistor Q3 for a high voltage and the N-type variable capacitance C1 can be formed at the same step and the extension regions of the PMOS transistor Q4 for a high voltage and the P-type variable capacitance C2 can be formed at the same step. Therefore, it is possible to obtain a semiconductor device having a variable capacitance with a series resistance component reduced while minimizing the number of manufacturing steps.

In some cases, moreover, the extension region of the transistor for a high voltage is to be formed as an LDD region to have an impurity concentration which is almost equal to that of the extension region of the high performance transistor. In these cases, an implantation energy is increased and the extension region is formed deeply.

Accordingly, it is possible to obtain the same effects, as those in the semiconductor device according to the sixth embodiment by forming the extension region of the variable capacitance comparatively deeply on the same conditions as those of the extension region of the transistor for a high voltage.

<Eighth Embodiment>

An eighth embodiment provides a semiconductor device having such a structure as to comprise a MOS transistor and a variable capacitance in which channel regions have different impurity concentrations, respectively. In the semiconductor device according to the eighth embodiment, the impurity concentrations of the channel regions in the MOS transistor and the variable capacitance are set to be different from each other so that a degree of freedom of a design in the device can be enhanced, for example, a threshold voltage can be set separately.

FIG. 20 is a sectional view showing a structure of the semiconductor device having the MOS transistor and the variable capacitance according to the eighth embodiment of the present invention. In FIG. 20, a PMOS transistor Q2 to be formed in a PMOS formation region A2 is the same as the PMOS transistor Q2 according to the first embodiment shown in FIG. 1.

On the other hand, an N-type variable capacitance C9 to be formed in an N-type variable capacitance formation region A3 has P³¹ pocket regions 37 in the vicinity of an extension portion 34 e and an N well region 31 provided between N⁺ extraction electrode regions 34 and 34 acts as a high concentration channel region 31 c. Other structures are the same as the structure of the N-type variable capacitance C1 according to the first embodiment shown in FIG. 1.

The N-type variable capacitance C9 includes the high concentration channel region 31 c having a higher N-type impurity concentration than that of other regions of the N well region 31. The high concentration channel region 31 c can cancel the P³¹ pocket regions 37 to sufficiently compensate for a reduction in a series resistance component. Therefore, a Q-value of the variable capacitance can be fully increased.

In the structure shown in FIG. 20, thus, when a channel concentration is to be changed between the PMOS transistor Q2 and the N-type variable capacitance C9, the high concentration channel region 31 c is provided in the N-type variable capacitance C9, thereby increasing the Q-value of the variable capacitance. More specifically, the structure shown in FIG. 20 is a desirable example in which impurity concentrations in the respective channel regions of the MOS transistor and the variable capacitance are set to be different from each other, thereby enhancing a degree of freedom of a design.

After the N well region 31 is formed, an N-type impurity is further implanted into an upper layer portion of the N well region 31 to obtain the high concentration channel region 31 c. More specifically, a step of forming the high concentration channel region 31 c is required separately.

While only the PMOS transistor and the N-type variable capacitance are shown in FIG. 20, it is a matter of course that an NMOS transistor and a P-type variable capacitance can also be formed to have the same structures.

<Ninth Embodiment>

FIG. 21 is a sectional view showing a part of a method of manufacturing a semiconductor device having a MOS transistor and a variable capacitance according to a ninth embodiment of the present invention.

In the method of manufacturing a semiconductor device according to the ninth embodiment, pocket regions are formed in both the MOS transistor and the variable capacitance.

It is assumed that source-drain regions are formed on the MOS transistor side and extraction electrode regions 34 are formed on the variable capacitance side by first ion implantation and diffusion through a forming step based on an existing method.

For the first impurity implantation and diffusion, a heat treatment (for example, RTA (Rapid Thermal Anneal) at 900 to 1100° C. for 10 to 120 seconds in an N₂ atmosphere) is carried out after implantation of an N-type impurity. For the heat treatment, a crystal defect formed by the implantation of the N-type impurity is recovered.

Subsequent processings are peculiar to the manufacturing method according to the ninth embodiment. In the ninth embodiment, furthermore, second impurity implantation and diffusion is not carried out over the MOS transistor but only the variable capacitance as shown in FIG. 21. In an example of FIG. 21, for the second impurity implantation, an N-type impurity ion 91 is implanted by using a gate electrode 33 as a mask to carry out a heat treatment. Consequently, N⁺ extraction electrode regions 34 h are formed so that an N-type variable capacitance C15 is finally obtained.

Examples of the second impurity implantation and diffusion include annealing to be carried out at a comparatively low temperature of 500 to 800° C. for approximately 10 to 120 minutes after the implantation of the N-type impurity.

The second impurity implantation and diffusion is carried out over only the variable capacitance and the heat treatment is performed at the comparatively low temperature during the diffusion as described above. Therefore, there is presented TED (Transient Enhanced Diffusion) to be a phenomenon in which a crystal defect formed by the second impurity ion implantation is introduced into a well region of the variable capacitance and a defect portion and an impurity are coupled to each other and are greatly diffused.

By the TED phenomenon, impurities in P³¹ pocket regions 37 and the N⁺ extraction electrode regions 34 h which are formed in the N-type variable capacitance C15 are diffused again. As a result, the presence of the P³¹ pocket regions 37 do not have such an influence that a series resistance is reduced. Consequently, it is possible to obtain the N-type variable capacitance C15 having a great Q-value.

While FIG. 21 shows the N-type variable capacitance C15, it is a matter of course that the present invention can also be applied to a P-type variable capacitance.

The second implantation may include a process of implanting an impurity ion of the N type in an oblique direction.

In this case, the deterioration in the resistance component by the pocket regions 37 can be effectively suppressed by the ion implantation in the oblique direction.

<Tenth Embodiment>

FIGS. 22 to 26 are sectional views showing a method of manufacturing an N-type variable capacitance in a semiconductor device having a MOS transistor and a variable capacitance according to a tenth embodiment of the present invention. With reference to these drawings, description will be given to a procedure for manufacturing the N-type variable capacitance according to the tenth embodiment.

As shown in FIG. 22, first of all, a lamination structure having a gate oxide film 32, a gate electrode 33 and an oxide film 59 for a mask is selectively provided on a surface of an N well region 31. By using the lamination structure as a mask, N- and P-type impurities are introduced to form an N³¹ extension region 38 and a P³¹ diffusion region 39, respectively. For a material of the gate electrode 33 to be formed, polysilicon is used.

As shown in FIG. 23, next, isotropic polysilicon etching is carried out over the gate electrode 33 to partially remove a peripheral region in a direction of a gate length of the gate electrode 33. Thus, a gate electrode 33 n having a small gate length is obtained.

As shown in FIG. 24, then, wet etching for an oxide film is carried out over the oxide film 59 for a mask and the gate oxide film 32. Consequently, an oxide film 59 n for a mask and a gate oxide film 32 n are obtained by reducing the oxide film 59 for a mask and the gate oxide film 32.

As shown in FIG. 25, then, a side wall 36 is formed on a side surface of the gate electrode 33 n.

As shown in FIG. 26, thereafter, an N-type impurity ion 75 is implanted and diffused by using the gate electrode 33 n and the side wall 36 as masks. Consequently, N⁺ extraction electrode regions 34 d are obtained. The N⁺ extraction electrode regions 34 d are formed in a region including a whole P³¹ diffusion region 39 and an N-type impurity concentration is higher than a P-type impurity concentration of the P³¹ diffusion region 39. Therefore, the influence of the P³¹ diffusion region 39 can be cancelled completely. More specifically, pocket regions are not present in the finished variable capacitance.

In the tenth embodiment, thus, the side wall is formed and the extraction electrode regions are provided after the gate length of the gate electrode is reduced. Consequently, the finished device can have such a structure that the pocket regions are not present. Therefore, it is possible to obtain an N-type variable capacitance having a great Q-value even if pocket regions forming step is included.

While the method of manufacturing the N-type variable capacitance has been described in the tenth embodiment, it is a matter of course that a P-type variable capacitance can be manufactured in the same manner.

<Eleventh Embodiment>

FIG. 27 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to an eleventh embodiment of the present invention. In FIG. 27, an NMOS transistor Q1 to be formed in an NMOS formation region A1 is the same as the NMOS transistor Q1 according to the first embodiment shown in FIG. 1.

On the other hand, an N-type variable capacitance C1 w to be formed in an N-type variable capacitance formation region A3 is different in that a thickness of a gate oxide film 32 w is greater than that of a gate oxide film 12. Other structures are the same as those of the N-type variable capacitance C1 according to the first embodiment shown in FIG. 1.

An oscillation frequency f of an LC type VCO is determined by the following equation (2). Therefore, it is desirable that a capacitance component of a variable capacitance should be reduced in order to fabricate an oscillator for oscillation at a high frequency.

[Equation  2] $\begin{matrix} {f = \frac{1}{2\pi\sqrt{LC}}} & (2) \end{matrix}$

However, there is a problem in that a series parasitic resistance is increased if a variable capacitance is manufactured in a small pattern.

As shown in FIG. 27, when the gate oxide film 32 w is formed to have a greater thickness than that of the gate oxide film 12, the capacitance component of the variable capacitance can be reduced without changing a pattern size, that is, increasing a parasitic resistance component. Furthermore, a Q-value can also be enhanced by a reduction in the capacitance component in accordance with the equation (1).

Moreover, in the case in which a transistor for a high voltage is provided in addition to a high performance transistor as in the semiconductor device according to the seventh embodiment, the gate oxide film 32 w is formed during formation of a gate oxide film of the transistor for a high voltage which has a greater thickness than that of a gate oxide film in the high performance transistor. Consequently, it is possible to obtain the gate oxide film 32 w having a smaller thickness than that of the high performance transistor without increasing the number of manufacturing steps.

While only the NMOS transistor and the N-type variable capacitance are shown in FIG. 27, it is a matter of course that a PMOS transistor and a P-type variable capacitance can also be formed to have the same structures.

<Twelfth Embodiment>

(First Mode)

FIG. 28 is a sectional view showing a structure of a first mode of a semiconductor device having a MOS transistor and a variable capacitance according to a twelfth embodiment of the present invention. In FIG. 28, an NMOS transistor Q1 to be formed in an NMOS formation region A1 is the same as the NMOS transistor Q1 according to the first embodiment shown in FIG. 1.

On the other hand, an N-type variable capacitance C1L to be formed in an N-type variable capacitance formation region A3 is different in that a material of a gate oxide film 32L has a lower dielectric constant than that of a material of a gate oxide film 12. Other structures are the same as those of the N-type variable capacitance C1 according to the first embodiment shown in FIG. 1.

In order to obtain the gate oxide film 32L, for example, it is proposed that F (fluorine) is implanted into only a gate oxide film 32 of the N-type variable capacitance C1L.

In the first mode of the twelfth embodiment, thus, a capacitance component of the variable capacitance can be reduced without increasing a parasitic resistance component. Therefore, the same effects as those in the eleventh embodiment can be obtained.

While only the NMOS transistor and the N-type variable capacitance are shown in FIG. 28, it is a matter of course that a PMOS transistor and a P-type variable capacitance can be formed to have the same structures.

(Second Mode)

FIG. 29 is a sectional view showing a structure of a second mode of the semiconductor device having a MOS transistor and a variable capacitance according to a twelfth embodiment of the present invention. In FIG. 29, an N-type variable capacitance C1 w to be formed in an N-type variable capacitance formation region A3 is different in that a thickness of a gate oxide film 32 w is greater than that of a gate oxide film 12. Other structures are the same as those of the N-type variable capacitance C1 according to the first embodiment shown in FIG. 1.

An NMOS transistor Q1 to be formed in an NMOS formation region A1 is different in that a gate insulating film 12H is formed by using a High-k material having a higher dielectric constant than that of a silicon oxide film and the gate insulating film 12H is formed to have an almost equal thickness to that of the gate oxide film 32 w. Other structures are the same as those of the NMOS transistor Q1 according to the first embodiment shown in FIG. 1.

Examples of the High-k material include Si₃N₄, Ta₂O₅, Al₂O₃, HfO₂, ZrO₂ and the like.

In the first mode of the twelfth embodiment, thus, it is possible to reduce a capacitance component of the variable capacitance without increasing a parasitic resistance component. Therefore, the same effects as those in the eleventh embodiment can be obtained.

The thickness of the gate insulating film 12H is almost equal to that of the gate oxide film 32 w. Therefore, the gate insulating film 12H and the gate oxide film 32 w can be manufactured at the same step. Consequently, it is possible to obtain the gate oxide film 32 w having a smaller thickness than that of a high performance transistor without increasing the number of manufacturing steps. In this case, since the gate insulating film 12H is formed of the High—k material, an electrical characteristic of an NMOS transistor Q1H is not adversely affected.

While only the NMOS transistor and the N-type variable capacitance are shown in FIG. 29, it is a matter of course that a PMOS transistor and a P-type variable capacitance can be formed to have the same structures.

<Thirteenth Embodiment>

FIG. 30 is a sectional view showing a structure of a semiconductor device having a MOS transistor and a variable capacitance according to a thirteenth embodiment of the present invention. In FIG. 30, an N-type variable capacitance C1 p to be formed in an N-type variable capacitance formation region A3 is different in that a porous silicon layer 8 is formed in an upper layer portion of an N well region 31. Other structures are the same as those of the N-type variable capacitance C1 according to the first embodiment shown in FIG. 1.

By the provision of the porous silicon layer 8, an effective dielectric constant of silicon is decreased so that a capacitance component of the N-type variable capacitance C1 p can be reduced. When a vacancy is continuously formed so that a rate (vacancy ratio) of the vacancy occupying the upper layer portion of the N well region 31 is too high, a resistance of the N well region 31 is increased. Therefore, it is desirable that the vacancy ratio should be 50% or less.

Since a capacitance component of the variable capacitance can be thus reduced without greatly increasing a parasitic resistance component in the thirteenth embodiment, the same effects as those in the eleventh embodiment can be obtained.

While only the N-type variable capacitance is shown in FIG. 30, it is a matter of course that a P-type variable capacitance can also be formed to have the same structure.

(Formation of Porous Silicon Layer)

FIGS. 31 to 35 are sectional views showing a method of forming a porous silicon layer which has been disclosed in Japanese Patent Application Laid-Open No. 2000-307112, for example. With reference to these drawings, a procedure for forming the porous silicon layer will be described below.

As shown in FIG. 31, first of all, a porous silicon layer 7 is formed in an upper surface of an N-type silicon substrate 6 by anode formation. More specifically, the silicon substrate 6 is immersed in an HF solution 152 in a formation layer 151 and a current is caused to flow to the silicon substrate 6 by setting an upper platinum electrode 153 to be a cathode and a lower platinum electrode 154 to be an anode. For conditions, a formation time of 30 seconds and a formation current density of 10 mA/cm² are set. As shown in FIG. 32, consequently, the upper surface of the silicon substrate 6 is made porous and the porous silicon layer 7 having a thickness of approximately 0.2 μm is formed in the upper surface of the silicon substrate 6.

FIG. 33 is a sectional view specifically showing a shape of the porous silicon layer 7. The porous silicon layer 7 has a complicated shape as shown in FIG. 33 (more specifically, see Document 2, pp 470, FIG. 4 or Document 3, pp 379, FIG. 2 which will be described below). In this specification, the shape of the porous silicon layer 7 is simplified for description as shown in FIG. 32. A thickness of the porous silicon layer 7 can be controlled based on a formation time and a formation current density, and furthermore, the vacancy ratio of the porous silicon layer 7 (a density corresponding to a ratio of a silicon portion 7 a to a vacancy portion 7 b) can be controlled by a concentration of the HF solution 152 (see SOI structure forming technique, pp 181 to 185, written by Seijiro Furukawa, 1987, Sangyo Tosho: (Document 1)).

In order to maintain stability of the porous structure of the porous silicon layer 7 for a heat treatment, next, preoxidation is carried out at a low temperature of approximately 400° C. In order to reduce the quantity of crystal defects of an expitaxial layer 9 to be formed at a subsequent step, then, the heat treatment is carried out at a temperature of 1000° C. or more for a few seconds in a hydrogen atmosphere. Consequently, a mobility of surface atoms is dramatically increased by minimization of a surface energy of the porous silicon layer 7 and a surface hole (not shown) generated in an upper surface of the porous silicon layer 7 due to natural oxidation of a surface is reduced and removed. As a result, as shown in FIG. 34, a porous silicon layer 8 is formed by sufficiently smoothening the upper surface of the porous silicon layer 7.

An upper surface of the porous silicon layer 8 maintains a single crystal structure of the silicon substrate 6 and has the same crystal orientation as that of the silicon substrate 6. As shown in FIG. 35, the epitaxial layer 9 having a thickness of approximately 100 nm is formed on the upper surface of the porous silicon layer 8 by an epitaxial growth method. See “Science of Silicon, pp 467–475, edited by Tadahiro Ohmi et al., REALIZE INC.” (Document 2), “IEICE TRANS. ELECTRON., VOL. E80-C, NO. 3, MARCH 1997, K. SAKAGUCHI et al., pp 378–387” (Document 3), and “Extended Abstracts of the 1998 International Conference on Solid State Devices and Materials, Hiroshima, 1998, pp 302–303” (Document 4) for the epitaxial growth of silicon on the porous silicon layer.

In the thirteenth embodiment, the porous silicon layer 8 is selectively formed in the N-type variable capacitance formation region A3 and a P-type variable capacitance formation region A4. Thus, in the case in which porous silicon is to be partially formed, surfaces of an NMOS formation region A1 and a PMOS formation region A2 are covered with a resist mask during the anode formation shown in FIG. 31 such that the porous silicon layer 7 is not formed.

While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. 

1. A semiconductor device comprising: a semiconductor substrate; an insulated gate type transistor formed in said semiconductor substrate; and an insulated gate type capacitor formed in said semiconductor substrate; said insulated gate type transistor including: a gate insulating film of the transistor selectively formed on said semiconductor substrate, a gate electrode of the transistor formed on said gate insulating film of the transistor, and source-drain regions formed to interpose a body region of the transistor provided under said gate electrode of the transistor in a surface of said semiconductor substrate; and said insulated gate type capacitor including: a gate insulating film of the capacitor selectively formed on said semiconductor substrate; a gate electrode of the capacitor formed on said gate insulating film of the capacitor; and extraction electrode regions formed to interpose a body region of the capacitor provided under said gate electrode of the capacitor in said surface of said semiconductor substrate, wherein said insulated gate type transistor has pocket regions of the transistor of a reverse conductivity type to that of said source-drain regions formed from said source-drain regions to a part of said body region of the transistor, said insulated gate type capacitor has no region of a reverse conductivity type to that of said extraction electrode regions in a vicinal region of said extraction electrode regions in said body region side of the capacitor, said source-drain regions include an extension portion of the transistor which has an upper layer portion protruded partially toward said body region side of the transistor, said extraction electrode regions include an extension portion of the capacitor which has an upper layer portion protruded partially toward said body region side of the capacitor, and said extension portion of the capacitor and said extension portion of the transistor being set to have impurity concentrations equal to each other.
 2. A semiconductor device comprising: a semiconductor substrate; an insulated gate type transistor formed in said semiconductor substrate; and an insulated gate type capacitor formed in said semiconductor substrate; said insulated gate type transistor including: a gate insulating film of the transistor selectively formed on said semiconductor substrate, a gate electrode of the transistor formed on said gate insulating film of the transistor, and source-drain regions formed to interpose a body region of the transistor provided under said gate electrode of the transistor in a surface of said semiconductor substrate; and said insulated gate type capacitor including: a gate insulating film of the capacitor selectively formed on said semiconductor substrate; a gate electrode of the capacitor formed on said gate insulating film of the capacitor; and extraction electrode regions formed to interpose a body region of the capacitor provided under said gate electrode of the capacitor in said surface of said semiconductor substrate, wherein said insulated gate type transistor has pocket regions of the transistor of a reverse conductivity type to that of said source-drain regions formed from said source-drain regions to a part of said body region of the transistor, said insulated gate type capacitor has no region of a reverse conductivity type to that of said extraction electrode regions in a vicinal region of said extraction electrode regions in said body region side of the capacitor, said source-drain regions include an extension portion of the transistor which has an upper layer portion protruded partially toward said body region side of the transistor, said extraction electrode regions include an extension portion of the capacitor which has an upper layer portion protruded partially toward said body region side of the capacitor, and said extension portion of the capacitor having a higher impurity concentration than that of said extension portion of the transistor.
 3. A semiconductor device comprising: a semiconductor substrate; an insulated gate type transistor formed in said semiconductor substrate; and an insulated gate type capacitor formed in said semiconductor substrate; said insulated gate type transistor including: a gate insulating film of the transistor selectively formed on said semiconductor substrate, a gate electrode of the transistor formed on said gate insulating film of the transistor, and source-drain regions formed to interpose a body region of the transistor provided under said gate electrode of the transistor in a surface of said semiconductor substrate; and said insulated gate type capacitor including: a gate insulating film of the capacitor selectively formed on said semiconductor substrate; a gate electrode of the capacitor formed on said gate insulating film of the capacitor; and extraction electrode regions formed to interpose a body region of the capacitor provided under said gate electrode of the capacitor in said surface of said semiconductor substrate, wherein said insulated gate type transistor has pocket regions of the transistor of a reverse conductivity type to that of said source-drain regions formed from said source-drain regions to a part of said body region of the transistor, said insulated gate type capacitor has no region of a reverse conductivity type to that of said extraction electrode regions in a vicinal region of said extraction electrode regions in said body region side of the capacitor, said insulated gate type transistor includes first and second transistors, said source-drain regions of said first transistor including a first extension portion of the transistor which has an upper layer portion protruded partially toward said body region side of the transistor, said source-drain regions of said second transistor including a second extension portion of the transistor which has an upper layer portion protruded partially toward said body region side of the transistor, said extraction electrode regions including an extension portion of the capacitor which has an upper layer portion protruded partially toward said body region side of the capacitor, and said extension portion of the capacitor having an impurity concentration set to be almost equal to that of said second extension portion of the transistor and to be higher than that of said first extension portion of the transistor.
 4. A semiconductor device comprising: a semiconductor substrate; an insulated gate type transistor formed in said semiconductor substrate; and an insulated gate type capacitor formed in said semiconductor substrate; said insulated gate type transistor including: a gate insulating film of the transistor selectively formed on said semiconductor substrate, a gate electrode of the transistor formed on said gate insulating film of the transistor, and source-drain regions formed to interpose a body region of the transistor provided under said gate electrode of the transistor in a surface of said semiconductor substrate; and said insulated gate type capacitor including: a gate insulating film of the capacitor selectively formed on said semiconductor substrate; a gate electrode of the capacitor formed on said gate insulating film of the capacitor; and extraction electrode regions formed to interpose a body region of the capacitor provided under said gate electrode of the capacitor in said surface of said semiconductor substrate, wherein said insulated gate type transistor has pocket regions of the transistor of a reverse conductivity type to that of said source-drain regions formed from said source-drain regions to a part of said body region of the transistor, said insulated gate type capacitor has no region of a reverse conductivity type to that of said extraction electrode regions in a vicinal region of said extraction electrode regions in said body region side of the capacitor, and said body region of the capacitor has a vacancy portion.
 5. A semiconductor device comprising: a semiconductor substrate; an insulated gate type transistor formed in said semiconductor substrate; and an insulated gate type capacitor which are formed in said semiconductor substrate; said insulated gate type transistor including: a gate insulating film of the transistor selectively formed on said semiconductor substrate; a gate electrode of the transistor formed on said gate insulating film of the transistor; source-drain regions formed to interpose a body region of the transistor provided under said gate electrode of the transistor in a surface of said semiconductor substrate; and pocket regions of the transistor of a reverse conductivity type to that of said source-drain regions formed from said source-drain regions to a part of said body region for a transistor; said insulated gate type capacitor includes: a gate insulating film of the capacitor selectively formed on said semiconductor substrate; a gate electrode of the capacitor formed on said gate insulating film of the capacitor; extraction electrode regions formed to interpose a body region of the capacitor provided under said gate electrode of the capacitor in said surface of said semiconductor substrate; and pocket regions of the capacitor of a reverse conductivity type to that of said extraction electrode regions, formed from said extraction electrode regions to a part of said body region of the capacitor, an impurity concentration in a surface of said body region of the capacitor being different from that in a surface of said body region of the transistor.
 6. The semiconductor device according to claim 5, wherein said impurity concentration in said surface of said body region of the capacitor is set to be higher than that in said surface of said body region of the transistor. 